Solid imaging devices fabricate three-dimensional objects from fusible powders or photocurable liquids, typically by exposure to radiation. Powders and liquids for solid imaging sometimes are referred to as “build materials” and the three-dimensional objects produced by solid imaging devices sometimes are called “builds,” “parts,” and “solid imaging products,” which can include a wide variety of shapes. Solid imaging includes many devices and methods to create three-dimensional objects, including by stereolithography, laser sintering, inkjet printing, and similar methods, which typically employ a layer-by-layer fabrication method. A laser or other source of radiation sequentially irradiates individual thin layers of the build material in response to which the material transforms to a solid, layer-upon-layer, to create a solid imaging product. More recent developments include flexible transport solid imaging devices and methods that are capable of using visible and ultraviolet light sources to irradiate build materials responsive to these wavelengths.
Despite the variety of devices and methods developed for solid imaging, a number of drawbacks have yet to be resolved. Typically, solid imaging devices produce products in batches rather than continuously or semi-continuously. Solid imaging devices produce “green” three-dimensional products, in which uncured build material wets the surface and causes the product to be tacky and to require cleaning prior to fully curing the product throughout the build.
Another drawback of many solid imaging devices is the limitation on the size of the objects that can be built. For machines that use a laser as the radiation source for transformation of build material, the time required to build the object is directly related to the object's size because of limitations on the speed with which a laser can scan the surface of the cross-section. Laser scanners can be provided with higher resolution capabilities and with a large size laser spot for scanning larger areas at once, completing borders and fine details with a smaller spot, but the scanning speed remains significantly slower than that which can be achieved with a digital light processing (“DLP”) imager.
DLP imagers employ mirror arrays in which selective control of each individual mirror between “on” and “off” positions produces the desired image in a fresh layer of photocurable build material. Each individual mirror correlates to a pixel, which is the smallest element of an image that can be individually processed in a display system for a two-dimensional image. DLP's and high resolution laser scanners have comparable minimum feature sizes. Imaging speed increases with a DLP as compared to a laser scanner because the individual mirrors in the DLP array can be controlled simultaneously to image an area of the build at one time, including an entire layer of a build, rather than by using a scanner to trace the image with a laser spot.
Systems using a DLP imager to produce larger objects have limitations. For larger objects, the DLP imager spreads the available radiation energy over a larger area, increasing the exposure time and reducing the resolution of the image. Commonly available imagers having a mirror array of 1024×768 pixels can expose a 9×6.75 inch area (22.9×17.14 cm area) in about 5 seconds and produce fine detail similar to that of a laser scanner. Increasing the image area to 18×13.5 inches (45.7×34.3 cm) increases exposure time to about 20 seconds and reduces fine detail twofold.
Image dimensions can be increased by 37% with the same minimum feature size by replacing a DLP having a 1024×768 pixel array with a higher resolution DLP having a 1400×1024 pixel array. That is to say, the same object could be produced with the same detail in a 12.2×9 inch format (31.0×22.9 cm format) with the larger array compared to the 9×6.75 inch format (22.9×17.14 cm format) of the smaller array. Nevertheless, higher resolution DLP imagers do not resolve the problems of increased exposure time and reduced detail for larger objects. The larger array reduces image intensity by 45% and increases exposure time from 5 to 9.1 seconds, which is significant over the course of a build.
Within limits, it should be possible to use higher intensity light sources with larger arrays to resolve some of the issues, but ever larger imagers and higher intensity light sources can be problematic to adapt for use in solid imaging devices. It would be desirable to improve the efficiency of solid imaging devices and methods and to produce three-dimensional objects in greater variety and with fewer restrictions.